Due to non-linear effects of analog components of a transmission chain, a transmitted signal is distorted in amplitude and phase. Such distortions usually depend on the signal magnitude.
The main cause for the distortions is a power amplifier of a transmitter. In addition to amplifying a desired signal, the power amplifier generates higher order harmonics of the original signal spectrum. The spread of the signal spectrum causes two major effects: a radio frequency spectrum mask does not fulfil the requirements for out-of-band radiated power, and detection of a distorted signal in a receiver suffers from errors.
The spread of the signal spectrum can be avoided (or at least diminished) by reducing the power of a power amplifier input signal. This is called “backing off”. However, such “backing off” leads to inefficient use of the amplification stage. Another option is to use a linearization technique. Several different prior art linearization techniques exist. The most effective of them are adaptive, since a plurality of factors, such as temperature, affect a transmission chain, making it unstable. Adaptation requires a reliable estimate of signal distortion. For the estimation, a feedback chain is typically used. A problem is that the feedback chain also generates distortion which sometimes makes the estimate of the signal distortion quite unreliable.